Zoantharia

Sawelew, Ludovic, Nuccio, Christopher, Foord, Colin, Lorquin, Jean & Perez, Yvan, 2022, Symbiodiniaceae diversity and characterization of palytoxin in various zoantharians (Anthozoa, Hexacorallia), Organisms Diversity & Evolution (New York, N. Y.) 22 (3), pp. 555-576 : 567-569

Zoantharia species identification

The unambiguous identification of a zoantharian species is a difficult task due to the lack of reliable diagnostic characters

◂ Fig. 5 Cross-sections of Palythoa sp. Z05. Sections at the mouth and tentacles (A), actinopharynx (C), and basal peduncle (E) levels, demonstrating the location and relative concentrations of palytoxin (PLTX) by MALDI-imaging mass spectrometry. Areas delimited by white rectangles are enlarged (B, D, F) and compared with images of the same histological regions exhibiting endogenous autofluorescence due to photosynthetic pigments of Symbiodiniaceae cells (G-I). Abbreviations: En, enteron; Ep, epidermis; Ph, pharynx; Se, septa; Te, tentacles. Colour scale, the highest PLTX concentrations (100%)−no PLTX (0%). Scale bars = 3 mm (A, C, E), 0.5 mm (B, D, F, G-I)

and high levels of intraspecific morphological variation mainly marked in their oral disk colour pattern and the form of the colony ( Burnett et al., 1997; Reimer et al., 2004, 2012). Six cases of intraspecific morphological plasticity were seen among the Palythoa and Zoanthus colonies used in this study: (1) P. aff. clavata Z26 and Z27; (2) P. mutuki Z06 and Z29; (3) P. heliodiscus Z03, Z04, and Z07; (4) Z. kuroshio Z14 and Z21; (5) Z. sociatus / sansibaricus Z15, Z17, Z23–Z25, and Z28; (6) Z. gigantus / solanderi Z08–Z13. In all cases, these zoantharians had either identical or very similar COI and ITS-rDNA sequences whereas colour phenotypes varied considerably (see supplementary informations, Fig. A). In addition to this, a previous phylogenetic study based on mitochondrial and nuclear sequences indicated that Z. vietnamensis and Z. kuroshio might be conspecific ( Reimer et al., 2006a).

Both our COI and ITS-rDNA tree patterns confirmed the previous topology of Zoanthus and Palythoa species, which displayed complexes of sibling species pairs between the Caribbean and Indo-Pacific regions ( Reimer et al., 2012). Indeed, in our results, the Palythoa species are clustered into three complexes: (1) P. caribaeorum / tuberculosa , (2) P. aff. clavata / sp. “sakurajimensis,” and (3) P. variabilis / heliodiscus . Zoanthus species were also divided into three complexes: (1) Z. pulchellus / kuroshio , (2) Z. sociatus / sansibaricus , and (3) Z. solanderi / gigantus . These sibling species pairs result from the closure of the Isthmus of Panama 3 million years ago ( Holcombe & Moore, 1977; O’Dea et al., 2016) and they consist of genetically identical or closely similar zoantharians that were previously named differently depending on their external morphology and location (Caribbean-Atlantic or Indo-Pacific regions) ( Reimer et al., 2012). P. grandis , known for its endemism in the Caribbean Sea, is the only sample that does not fit into any of the complexes listed above and does not have any recorded sister species in the Indo-Pacific basin. These findings are congruent with those of Reimer et al. (2012). Finally, our molecular analyses showed that Palythoa sp. Z05 is a putatively undescribed Indo-Pacific species. Its closest relative is P. aff. clavata found in Florida waters ( Reimer et al., 2012). The morphology of Palythoa sp. Z05 does not match conclusively with any Indo-Pacific described species but displays striking similarities with a specimen from the Cape Verde Islands named Palythoa sp. 265 ( Reimer et al., 2010) which is also genetically very close to P. aff. clavata ( Reimer et al., 2012) . Later, Reimer et al. (2014) identified new specimens from Ascension Island belonging to the clavata complex. Both COI and ITS-rDNA markers have shown that the undescribed Indo-Pacific species P. sp. “sakurajimensis” is close to Palythoa sp. Z05. However, this last result is not congruent with morphological data, emphasizing once more that, in zoantharians, genetic similarities do not necessarily reflect morphological ones. In general, species boundaries are primarily based on morphology and it is usually assumed that morphological variations reflect reproductive isolation along with genetic differentiation. However, genetic studies and fertilization trials producing viable larvae suggest that morphological and genetic distinctions do not always correlate in corals (for a review see Miller & Benzie, 1997). Hybridization on coral reefs is common and widespread (for a review see Richards & Hobbs, 2015). So far, even if it is reasonable to consider Palythoa sp. Z05 as an undescribed species based on its morphology and location of collection, it is not possible to assess to which extent hybridization with colonies of P. sp. “sakurajimensis” occurs naturally on the reef.